This invention relates to an in-focus detecting device which, in an optical video disk player or the like, detects whether or not a reading device is correctly positioned in the control range of a focus servo device adapted to maintain a predetermined distance between the reading device (or a focus lens) and a disk.
In a conventional device of this type, the video carrier signal recorded in the disk is read out to detect whether or not the output level of the signal thus read reaches a predetermined value. This is used to provide an in-focus signal and an out-focus signal.
However, the conventional device is disadvantageous in the following respects. In the conventional device, the output level of the video carrier signal changes with the states of the disk, light source and light receiving element. The output level is not always in correspondence to the focus condition of the reading device. Therefore, sometimes erroneous operation results where, although the reading device is out of the control range of the focus servo device, the in-focus signal is produced. Furthermore, in the case where the in-focus or out-focus signal is provided at the output terminal of the focus control amplifier, a signal from the light receiving means is delayed before reaching the focus control amplifier.
Also, when the focus servo device becomes out of order for some reason, or variation occurs beyond its control limit, the focus lens may be brought into contact with the disk. This difficulty may be overcome by providing a particular protecting device which is operated by using the output of the focus control amplifier. However, this method is disadvantageous in that signals from the light receiving elements are delayed before reaching the focus control amplifier.